1. Field of the Invention
This invention relates a microwave coupler. In particular, the invention relates to a compensated mixed dielectric overlay coupler which corrects mismatched mode velocities.
2. Description of the Prior Art
A suspended stripline is a convenient medium to use for microwave components which need the low loss properties of an air dielectric stripline, but with the convenience of fabrication possible with printed circuit conductors. FIG. 1 shows the configuration of a known suspended stripline transmission line 10. In the usual case, the substrate 12, which is a relatively thin low dielectric material of thickness t carries stripline conductor 14. A ground plane 16 is spaced by a distance d from the substrate 12. The spacing d is large compared to the thickness t. In the case illustrated, the transmission line represented by the conductor 14 and the ground plane 16 has essentially the same impedance, loss, and wave velocity as if the conductor were embedded in pure air. Likewise, the substrate 12 may be suspended between strips of "mostly air" honeycomb material, not shown, and formed into a lightweight bonded unit which retains most of the useful properties of an air dielectric stripline.
A known overlay coupler 20 shown in FIG. 2 employs opposed striplines 22 and is a common arrangement using pure stripline forms. The optimum length of the coupled lines 22 (into the page) is .lambda./4 at the design center frequency. The coupler directivity is theoretically perfect over a wide bandwidth. Most commercial hybrid couplers are manufactured in this manner, usually with air forming the dielectric between, above and below the strips 22.
FIGS. 3A and 3B show a known overlay coupler 24 employing striplines 26 on a suspended substrate 28 operating in respective even and odd modes. This type of coupler has not previously been considered advantageous or particularly useful because the dielectric constant of the substrate 28 between the striplines 26 is different from the dielectric constant of the air in the spaces 29 above and below the striplines 26 resulting in different wave velocities in the even and odd modes. The overlay coupler 24 does not work well because the odd mode transmission medium (substrate 28) has a different effective dielectric constant from the effective even mode medium (air space 29).
FIGS. 3A and 3B illustrate why the even and odd modes operation of the suspended stripline overlay coupler 24 have different effective dielectric constants, and therefore different wave velocities. In the even mode, FIG. 3A, the striplines 26 are the same polarity. The electric field E is confined in the spaces 29 between the strips 26 and ground planes 16. There is essentially no field between the strips 26 in the dielectric substrate 28, and the effective even mode wave propagation is very similar to an air stripline, with the strip width W and a thickness S as shown.
In the odd mode, FIG. 3B, the striplines are opposite polarity and the field E is concentrated between the strips in the dielectric substrate 28. In fact, there is a virtual ground G.sub.v which can be constructed by symmetry between the strips 26, and the effective odd mode wave propagation is essentially that of a microstrip with a dielectric substrate of thickness S/2. Because the even and odd modes operate in media having vastly different effective dielectric constants, the propagation velocities are also different and it is not possible to choose a coupler length which is a quarter wave of both modes. As a result, no matter what length is chosen, either or both the even or odd modes effective lengths will not be .lambda./4, and the result is a coupler having poor directivity and impedance match.